1. Field of the Invention
The present invention pertains to electron and ion beam columns, and, more particularly, to shielding of such columns from electromagnetic (EMI, RFI) interference.
2. Description of Related Art
Electron and ion beam columns use electrical lenses and scan coils, together with mechanical shields, to focus the electrons or ions in the column. In the areas of the column which the beam passes that are not in close proximity to a lense or a coil, the beam is subject to external electrical interference such as EMI, RFI, etc. Also subject to interference are the column control signals.
In addition, the control of these beams is becoming increasingly more stringent as geometries of integrated circuits are becoming smaller. For example, one use for an ion beam is to open a vertical connection in an integrated circuit by removing a vertical conductive region. With IC geometries used today and prior art ion beam systems, the beam must be accurate to within about 0.2 micrometers (.mu.m) and must hold this accuracy during the entire operation which takes over 20 minutes. Typically, electron/ion beam system manufacturers do not guarantee image drift for as long as 20 minutes.
Therefore, it can be appreciated that preventing unwanted electrical noise from interfering with an electron or ion beam is highly desirable.